The fabrication of ceramic, metal and other shaped articles by sintering powder compacts, whether formed by die pressing, slip casting, or injection molding, involves a series of certain basic steps. The raw sinterable powder, such as ceramic or metal powder is first mixed with a binder and sometimes a suitable solvent. The binder-powder mixture is then formed or shaped and any solvent, if present, is removed. In the final step the powder compact is heated under conditions which lead to elimination of the binder material and any solvent that may still have been present. Continued heating results in the consolidation of the individual grains of the sinterable powder, such as ceramic, plastic or metal powder and the creation of a solid monolithic object that is free of voids.
Although processes such as die pressing, slip casting, and injection molding are capable of delivering finished objects of great complexity in a bewildering array of materials, they all suffer from one major drawback. These processes all involve forming compacted powders into complex shapes. Powder compacts of these materials invariably have numerous voids between the individual constituent powder grains of the compacted powder. Typically such compacted powder objects contain only from about 40% to 65% solids, by volume, before heating. Voids amounting to 35% to 60% constitute the remainder. When such compacts are heated, the voids are eliminated, and the powder compact undergoes linear shrinkage on the order of 15%-25%.
When an accurate replica of a shaped article is sought in terms of either or both size and proportional configuration, it is necessary to oversize the powder compact to an extent sufficient to offset the shrinkage which occurs during the heating of the powder compact, which is the last step of the procedure.
By way of illustration, if a precise dimensional replica of a shaped article is desired when using an injection molding process, the injection mold die cavities must be enlarged by an amount equal to the amount of shrinkage that will take place when the molded powder is heated. The same type of enlargement must be made to compensate for shrinkage on heating in all powder forming processes.
The dimensional compensation needed for accurate replica of shaped articles can be provided by hand enlargement of a mold, produced from the article to be replicated. However, this solution to the problem is obviously labor intensive and requires great skill to produce a proportionately accurate oversized mold. Since the careful crafting of the oversized molds is obviously time consuming and costly, the use of this technique is usually limited, either to replicas where the high cost of the finished article is not a deterrent or to the production of large numbers of identically shaped articles. These are to be distinguished from manufacturing situations where only a few relatively low valued replicas are to be made. In addition to being highly labor intensive, the accuracy of sculpting to provide shrinkage compensation for complex objects is completely dependent upon the skill of the artisan. As observed by Randall German in the book "Powder Injection Molding", p.255, tool design can involve using the ". . . costly trial-and-error approach."
The problem of shrinkage resulting from heating can be avoided, in some instances if it is acceptable for the replicated article to be produced from materials that exhibit only a slight amount of shrinkage. For example molten metals or glass can be cast in a mold with very little resulting shrinkage, but there are only a very limited number of circumstances where these materials are acceptable for the final replicated article. There also have been attempts to develop special ceramics which undergo a phase change and some degree of expansion during heating to thereby counteract the volume contraction which would otherwise result from the elimination of voids. Unfortunately these materials have found only limited use, because other essential properties of the final product, such as thermal expansion, strength, fracture toughness, etc. are not adequate for most applications.